1. Field of the Invention
This invention relates to a light emission device, which is utilizable in various fields as a variety of display devices, and more particularly, to an organic electroluminescent device, which works at a low drive voltage and is high brightness and excellent in stability.
2. Description of the Prior Art
Electroluminescent devices are self-luminescent in nature, and are thus brighter than light crystal devices, enabling a clear display. Accordingly, research workers have long studied such devices. Existing electroluminescent devices, which arrive at a level in practical use, are those which make use of inorganic materials such as ZnS. However, such inorganic electroluminescent devices are not in wide use because their drive voltage for emission is as high as 200 V or over.
On the contrary, organic electroluminescent devices, which utilize light-emitting organic materials, have still been far from the practical level. In 1987, C. W. Tang et al of Eastman Kodak Co., (Applied Physics Letters, Vol. 51, p. 918, 1987) developed a builtup structure device, with a drastic advance in characteristic performance. More particularly, they succeeded in light emission by use of a builtup structure which includes a phosphor or fluorescent body having the stable structure of a deposition film and capable of transporting electrons, and an organic film capable of transporting holes. The carriers from both are injected into the phosphor body. This organic electroluminescent device has a much improved luminous efficiency, and is capable of emission at 1000 cd/m2 or more on application of a voltage of 10V or below.
Furthermore, as set out in Journal of Applied Physics, Vol. 65, p. 3610, 1989), C. W. Tang et al of Eastman Kodak Co., proposed an emission layer using a guest/host system, making it possible to further improve a luminous efficiency and use a diversity of luminescent materials. At present, most of the organic electroluminescent devices having a high luminous efficiency employ an electroluminescent layer based on the guest/host system.
However, in order to obtain a device whose luminous efficiency is improved by use of the guest/host system, it is necessary that a light-emitting material serving as a guest material be doped at a low concentration of approximately 1 mole % in a host material. The light emission layer of the organic electroluminescent device is approximately several tens of nanometers in thickness. The emission layer is usually formed by vacuum deposition. Accordingly, a difficulty is involved in the uniform formation of a thin film emission layer, doped at such a low concentration, on a substrate with a great area. This presents a serious problem on the application of the device using the gust/host system.
In addition, when a light-emitting material used as a guest material is employed as an emission layer in a usual manner, it has been difficult to obtain an organic electroluminescent device with a good luminous efficiency.
It is accordingly an object of the invention to provide an organic electroluminescent device which is high in luminous efficiency and is capable of realizing light emission in different colors and whose brightness lowers only slightly over the life of the device.
It is another object of the invention to provide an organic electroluminescent device which has a very thin emission layer or islands made of an organic material whereby light emission is achieved by use of the organic material alone or in admixture with other material in high concentration, which has never been experienced in prior art.
It is a further object of the invention to provide an organic electroluminescent device which has several emission layers as separate layers in the device whereby a higher luminous efficiency is realized.
It is a still further object of the invention to provide an organic electroluminescent device, which is able to emit white light.
Broadly, the invention provides an organic electroluminescent device, which comprises a pair of electrodes, and a layer structure provided between the pair of electrodes and including a charge transport layer capable of transporting electrons or holes and an electroluminescent or emission layer comprising at least 50 wt % of an organic material capable of emitting light on application of a voltage thereto via the pair of electrodes, wherein the organic material undergoes xe2x80x9cconcentration quenching and the emission layer has a thickness of 4 nm or below. The term xe2x80x9cconcentration quenchingxe2x80x9d used herein is intended to mean the phenomenon that when the concentration of the material increases, emission ceases. In the practice of the invention, there are used organic compounds, which allow concentration quenching to occur at a concentration of 10 mole % or over in a layer composition. The concentration may, more or less, vary depending on the type of luminescent material.
Preferably, the emission layer consists essentially of the organic material.
Alternatively, the organic material used in the invention may be one, which may or may not undergo any xe2x80x9cconcentration quenchingxe2x80x9d, but has a fluorescence lifetime shorter than a constituent organic material for the charge transport layer. In this case, the emission layer should also have a thickness of 4 nm or below. More particularly, there is also provided, according to another embodiment of the invention, an organic electroluminescent device, which comprises a pair of electrodes, and a layer structure provided between the pair of electrodes and including a charge transport layer capable of transporting electrons or holes and an emission layer consisting essentially of an organic material capable of emitting light on application of a voltage thereto via the pair of electrodes, wherein the organic material has a fluorescent lifetime shorter than that of an organic material present in the charge transport layer.